Method for micro-fabricating a pixelless infrared imaging device

ABSTRACT

The present invention relates to a method for micro-fabricating a pixelless thermal imaging device. The imaging device up-converts a sensed 2-dimensional M/FIR image into a 2-dimensional image in the NIR to visible spectrum in dependence thereupon. A plurality of layers forming an integrated QWIP-LED wafer are crystallographically grown on a surface of a first substrate. The layers comprise an etch stop layer, a bottom contact layer, a plurality of layers forming a QWIP and a LED, and a top contact layer. At the top of the QWIP-LED wafer an optical coupler such as a diffraction grating for coupling at least a portion of incident M/FIR light into modes having an electric field component perpendicular to quantum wells of the QWIP is provided. In following processing steps the first substrate and the etch stop layer are removed. Various different thermal imaging devices are manufactured by changing the order of manufacturing steps, omitting some steps or using different materials. Therefore, it is possible using a same manufacturing equipment for producing a large variety of different imaging devices considerably reducing manufacturing costs.

This application claims the benefit of U.S. Provisional PatentApplication No. 60/289,521 filed May 09, 2001.

FIELD OF THE INVENTION

This invention relates to infrared thermal imaging devices and inparticular to micro-fabrication of pixelless infrared thermal imagingdevices comprising epitaxially integrated quantum well infraredphotodetector and light emitting diode.

BACKGROUND OF THE INVENTION

Infrared imaging is widely used in a variety of applications includingnight vision, surveillance, search and rescue, remote sensing, andpreventive maintenance, to name a few. Imaging devices to provide theseapplications are typically constructed of HgCdTe or InSb focal planearrays. These focal plane arrays are known to be pixel mapped devices,where an array element is generally mapped to one or more circuitelements. However, such focal plane arrays are difficult to manufactureand expensive. Quantum Well Infrared Photodetectors (QWIPs) are able todetect Mid to Far InfraRed (M/FIR) light, providing an output current asa result. However, such devices have not been able to be successfullyused in efficient and inexpensive image detectors. The basic idea ofQWIPs using intraband or intersub-band transition for M/FIR detectionhave been disclosed in U.S. Pat. No. 4,205,33 1, issued May 27, 1980 toEsaki et al. and in U.S. Pat. No. 4,873,555, issued Oct. 10, 1989, toCoon et al. Embodiments of QWIPs using intraband or intersubbandtransitions have been disclosed in U.S. Pat. No. 4,894,526, issuedJan.16, 1990, to Bethea et al. and U.S. Pat. No. 5,023,685, issued Jun.11, 1991 to Bethea et al. The latter two patents describe a devicehaving improved efficiency by utilizing a series of quantum wells.

An improvement of these earlier technologies was disclosed by one of thepresent inventors, H. C. Liu, in U.S. Pat. No. 5,567,955, issued Oct.22, 1996, to the National Research Council of Canada, incorporatedherein by reference, wherein the vertical integration of a LightEmitting Diode (LED) with a QWIP is described. The QWIP-LED is a photonfrequency up-conversion device. The device comprises either aphoto-diode or a photo-conductor connected in series with a LED. Thephoto-diode or the photo-conductor acts as a M/FIR detector, whereas theLED emits in the NIR or the visible spectrum. A forward constant bias isapplied to the LED with respect to the QWIP. A M/FIR excitation of thedetector decreases its resistance and thereby increases the voltagedropped across the LED, leading to an increase in the LED emissionintensity. Therefore, the incoming M/FIR radiation has been convertedinto an increase of the NIR or visible emission. The emission in the NIRis efficiently detected by a Si Charge-Couple Device (CCD), resulting ina highly efficient detector. The vertical integration results fromepitaxial deposition of the LED material over the QWIP materials.

Details about the QWIP-LED technology as well as numerous embodimentsare disclosed in the following references incorporated herein byreference:

U.S. Pat. No. 5,646,421, issued Jul. 8, 1997, to H. C. Liu;

U.S. Pat. No. 6,028,323, issued Feb. 22, 2000, to H. C. Liu;

H. C. Liu, L. B. Allard, M. Buchanan, Z. R. Wasilewski, “Pixellessinfrared imaging device”, Electronics Letters 33, 5 (1997);

L. B. Allard, H. C. Liu, M. Buchanan, Z. R. Wasilewski, “Pixellessinfrared imaging utilizing a p-type quantum well infrared photodetectorintegrated with a lightemitting diode”, Appl. Phys. Lett. 70, 21 (1997);

E. Dupont, H. C. Liu, M. Buchanan, Z. R. Wasilewski, D. St-Germain, P.Chevrette, “Pixelless infrared imaging devices based on the integrationof n-type quantum well infrared photodetector with near-infrared lightemitting diode”, (Photonics West, San Jose, January 1999), SPIE Proc.3629, 155 (1999);

E. Dupont, H. C. Liu, M. Buchanan, S. Chiu, M. Gao, “Efficient GaAslight-emitting diods by photon recycling”, Appl. Phys. Lett. 76, 1(2000);

E. Dupont, S. Chiu, “Efficient light-emitting diodes by photon recyclingand their application in pixelless infrared imaging devices”, J. Appl.Phys. 87, 1023, (2000);

S. Chiu, M. Buchanan, E. Dupont, C. Py, H. C. Liu, “Substrate removalfor improved performance of QWIP-LED devices grown on GaAs substrates”,Infrared Phys. And Techn. 41, 51 (2000); and,

E. Dupont, M. Gao, Z. Wasilewski, H. C. Liu, “Integration of n-type andp-type quantum well infrared photodetectors for sequential multicoloroperation”, Appl. Phys. Lett. 78, 14 (2001).

A pixelless thermal imaging device is achieved by a suitably fabricatedQWIP-LED having a sufficiently large active area for the detection of a2-dimensional M/FIR image. The up-conversion device is made sufficientlylarge in area for sensing a 2-dimensional M/FIR image, and an emitted2-dimensional image in the NIR or visible spectrum is then detected by astandard Si CCD or other standard imaging device. It is possible tomanufacture large format 2-dimensional thermal imaging devices having aperfect fill factor without the need for complex readout circuits. Theintegrated QWIP-LED technology allows manufacture of efficient andinexpensive thermal imaging devices.

It is, therefore, an object of the invention to provide amicro-fabrication method for manufacturing efficient and inexpensivepixelless infrared thermal imaging devices.

It is further an object of the invention to provide a micro-fabricationmethod for manufacturing pixelless infrared thermal imaging devicesbased on epitaxial integration of a QWIP with a LED.

It is yet another object of the invention to provide a micro-fabricationmethod for manufacturing pixelless infrared thermal imaging devicesallowing use of a same manufacturing equipment for producing a largevariety of different devices.

SUMMARY OF THE INVENTION

The micro-fabrication method according to the invention allowsmanufacture of numerous different infrared imaging devices based onepitaxial integration of a QWIP with a LED. The various steps of themicro-fabrication method are based on standard manufacturing techniques,for example, epitaxial growth and etching. Furthermore, variousdifferent devices are manufactured by changing the order ofmanufacturing steps, omitting some steps or using different materials.Therefore, it is possible using a same manufacturing equipment forproducing a large variety of different imaging devices considerablyreducing manufacturing costs.

In accordance with the present invention there is provided a method formicro-fabricating a pixelless thermal imaging device, the imaging devicefor up-converting a sensed 2-dimensional M/FIR image into a2-dimensional image in the NIR to visible spectrum in dependencethereupon, the method comprising the steps of:

providing a first substrate, the first substrate having a surfacesuitable for subsequent crystal growth;

crystallographically growing an integrated QWIP-LED wafer on the surfaceof the first substrate comprising the steps of:

growing an etch stop layer;

growing a bottom contact layer;

growing a plurality of layers forming a n-type QWP and a LED; and,

growing a top contact layer;

providing at the top of the QWIP-LED wafer an optical coupler forcoupling at least a portion of incident M/FIR light into modes having anelectric field component perpendicular to quantum wells of the QWIP;

removing the first substrate; and,

removing the etch stop layer.

In accordance with an aspect of the present invention there is provideda method for micro-fabricating a pixelless thermal imaging device, theimaging device for up-converting a sensed 2-dimensional M/FIR image intoa 2-dimensional image in the NIR to visible spectrum in dependencethereupon, the method comprising the steps of:

providing a first substrate, the first substrate having a surfacesuitable for subsequent crystal growth;

crystallographically growing an integrated QWIP-LED wafer on the surfaceof the first substrate comprising the steps of:

growing an etch stop layer;

growing a bottom contact layer;

growing a plurality of layers forming a n-type QWIP and a LED; and,

growing a top contact layer;

providing an optical coupler on the top of the QWIP-LED wafer forcoupling at least a portion of incident M/FIR light into modes having anelectric field component perpendicular to quantum wells of the n-typeQWIP;

patterning a device mesa by removing the layers outside the device mesadown to the bottom contact layer, the device mesa approximatelycomprising an active area of the thermal imaging device, the active areabeing approximately the size of the 2-dimensional image;

depositing a top metal contact onto the top contact layer such that thetop metal contact forms a ring surrounding the active area;

depositing a bottom metal contact onto the bottom contact layer outsidethe device mesa;

depositing a coating onto the top surface of the active area;

isolating material defects in the active area of the QWIP-LED;

bonding the QWIP-LED wafer to an optical faceplate such that theQWIP-LED is in optical communication with the optical faceplate forlight emitted from the LED;

removing the first substrate; and,

removing the etch stop layer.

In accordance with the present invention there is further provided amethod for micro-fabricating a pixelless thermal imaging device, theimaging device for up-converting a sensed 2-dimensional M/FIR image intoa 2-dimensional image in the NIR to visible spectrum in dependencethereupon, the method comprising the steps of:

providing a first substrate, the first substrate having a surfacesuitable for subsequent crystal growth;

crystallographically growing on the surface of the first substrate aplurality of layers forming an integrated QWIP-LED wafer;

patterning a device mesa, the device mesa approximately comprising anactive area of the thermal imaging device, the active area beingapproximately the size of the 2-dimensional image; and,

isolating material defects in the active area of the QWIP-LED.

In accordance with another aspect of the present invention there isprovided a method for micro-fabricating a pixelless thermal imagingdevice, the imaging device for up-converting a sensed 2-dimensionalM/FIR image into a 2-dimensional image in the NIR to visible spectrum independence thereupon, the method comprising the steps of:

providing a first substrate, the first substrate having a surfacesuitable for subsequent crystal growth;

crystallographically growing an integrated QWIP-LED wafer on the surfaceof the first substrate comprising the steps of:

growing an etch stop layer;

growing a bottom contact layer;

growing a plurality of layers forming a n-type QWIP and a LED; and,

growing a top contact layer;

patterning a device mesa by removing the layers outside the device mesadown to the bottom contact layer, the device mesa approximatelycomprising an active area of the thermal imaging device, the active areabeing approximately the size of the 2-dimensional image;

depositing a top metal contact onto the top contact layer such that thetop metal contact forms a ring surrounding the active area;

depositing a bottom metal contact onto the bottom contact layer outsidethe device mesa;

isolating material defects in the active area of the QWIP-LED;

bonding the top surface of the QWIP-LED wafer to an optical faceplatesuch that the QWIP-LED is in optical communication with the opticalfaceplate for light emitted from the LED;

removing the first substrate;

removing the etch stop layer;

providing an optical coupler at the bottom of the QWIP-LED wafer forcoupling at least a portion of incident M/FIR light into modes having anelectric field component perpendicular to quantum wells of the n-typeQWIP; and,

bonding the bottom surface of the QWIP-LED wafer to a plate such thatthe QWIP-LED is in optical communication with the plate for M/FIR light.

BRIEF DESCRIPTION OF THE FIGURES

Exemplary embodiments of the invention will now be described inconjunction with the following drawings, in which:

FIG. 1 is a simplified block diagram of a thermal imaging devicefabricated using a method for micro-fabricating a pixelless thermalimaging device according to the invention;

FIG. 2 is a simplified flow diagram of a method for micro-fabricating apixelless thermal imaging device according to the invention;

FIG. 3a is a simplified block diagram schematically illustrating adevice structure obtained after manufacturing step a) shown in FIG. 2;

FIG. 3b is a simplified block diagram schematically illustrating adevice structure obtained after manufacturing step b) shown in FIG. 2;

FIG. 3c is a simplified block diagram schematically illustrating adevice structure obtained after manufacturing step c) shown in FIG. 2;

FIG. 3d is a simplified block diagram schematically illustrating adevice structure obtained after manufacturing step d) shown in FIG. 2;

FIG. 3e is a simplified block diagram schematically illustrating adevice structure obtained after manufacturing step e) shown in FIG. 2;

FIG. 3f is a simplified block diagram schematically illustrating adevice structure obtained after manufacturing step f) shown in FIG. 2;

FIG. 3g is a simplified block diagram schematically illustrating adevice structure obtained after manufacturing step g) shown in FIG. 2;

FIG. 3h is a simplified block diagram schematically illustrating adevice structure obtained after manufacturing step h) shown in FIG. 2;

FIG. 3i is a simplified block diagram schematically illustrating adevice structure obtained after manufacturing step i) shown in FIG. 2;

FIG. 3j is a simplified block diagram schematically illustrating adevice structure obtained after manufacturing step j) shown in FIG. 2;

FIG. 3k is a simplified block diagram schematically illustrating adevice structure obtained after manufacturing step k) shown in FIG. 2;

FIG. 3l is a simplified block diagram schematically illustrating adevice structure obtained after manufacturing step l) shown in FIG. 2;

FIG. 3m is a simplified block diagram schematically illustrating adevice structure obtained after manufacturing step m) shown in FIG. 2;

FIG. 3n is a simplified block diagram schematically illustrating adevice structure obtained after manufacturing step n) shown in FIG. 2;

FIG. 4 is a simplified block diagram schematically illustrating thestructure of a n-type QWIP-LED wafer micro-fabricated using a methodaccording to the invention;

FIG. 5a is a simplified block diagram schematically illustrating theincorporation of a QWIP-LED wafer micro-fabricated using a methodaccording to the invention in a thermal imaging device operating in areflective mode;

FIG. 5b is a simplified block diagram schematically illustrating theincorporation of a QWIP-LED wafer micro-fabricated using a methodaccording to the invention in a thermal imaging device operating in atransmissive mode;

FIG. 6 is a simplified block diagram schematically illustrating thestructure of a p-type QWIP-LED wafer micro-fabricated using a methodaccording to the invention;

FIG. 7 is a simplified block diagram schematically illustrating thestructure of another n-type QWIP-LED wafer micro-fabricated using amethod according to the invention;

FIG. 8 is a simplified block diagram schematically illustrating thestructure of a n-type QWIP/LED/p-type QWIP wafer micro-fabricated usinga method according to the invention;

FIG. 9a is a simplified block diagram schematically illustrating a crosssectional view of a diffractional grating;

FIG. 9b is a simplified block diagram schematically illustrating a topview of the diffractional grating shown in FIG. 9a;

FIG. 10a is a simplified block diagram schematically illustrating across sectional view of a lamellar V-groove structure;

FIG. 10b is a simplified block diagram schematically illustrating a topview of the lamellar V-groove structure shown in FIG. 10a;

FIG. 11 is a simplified block diagram schematically illustrating an-type QWIP-LED with a grating and reflective coating on the top surfacemicro-fabricated using a method according to the invention;

FIG. 12a is a simplified block diagram schematically illustrating amaterial defect in a QWIP-LED;

FIG. 12b is a simplified block diagram schematically illustratingisolation of the material defect in a QWIP-LED shown in FIG. 12a usingshort pulse laser ablation as a processing step in a method according tothe invention;

FIG. 12c is a simplified block diagram schematically illustratingisolation of the material defect in a QWIP-LED shown in FIG. 12a usingshort pulse laser ablation as a processing step in a method according tothe invention;

FIG. 13 is a simplified flow diagram of another method formicro-fabricating a pixelless thermal imaging device according to theinvention; and,

FIG. 14a is a simplified block diagram schematically illustrating apixelless infrared imaging device manufactured using the methods formicro-fabrication according to the invention;

FIG. 14b is a simplified block diagram schematically illustrating apixelless infrared imaging device manufactured using the methods formicro-fabrication according to the invention;

FIG. 14c is a simplified block diagram schematically illustrating apixelless infrared imaging device manufactured using the methods formicro-fabrication according to the invention;

FIG. 14d is a simplified block diagram schematically illustrating apixelless infrared imaging device manufactured using the methods formicro-fabrication according to the invention;

FIG. 14e is a simplified block diagram schematically illustrating apixelless infrared imaging device manufactured using the methods formicro-fabrication according to the invention;

FIG. 14f is a simplified block diagram schematically illustrating apixelless infrared imaging device manufactured using the methods formicro-fabrication according to the invention;

FIG. 14g is a simplified block diagram schematically illustrating apixelless infrared imaging device manufactured using the methods formicro-fabrication according to the invention;

FIG. 14h is a simplified block diagram schematically illustrating apixelless infrared imaging device manufactured using the methods formicro-fabrication according to the invention;

FIG. 14i is a simplified block diagram schematically illustrating apixelless infrared imaging device manufactured using the methods formicro-fabrication according to the invention;

FIG. 14j is a simplified block diagram schematically illustrating apixelless infrared imaging device manufactured using the methods formicro-fabrication according to the invention;

FIG. 14k is a simplified block diagram schematically illustrating apixelless infrared imaging device manufactured using the methods formicro-fabrication according to the invention;

FIG. 14l is a simplified block diagram schematically illustrating apixelless infrared imaging device manufactured using the methods formicro-fabrication according to the invention; and,

FIG. 14m is a simplified block diagram schematically illustrating apixelless infrared imaging device manufactured using the methods formicro-fabrication according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates schematically an example of a completed pixelles IRthermal imaging device based on an epitaxially integrated QWIP-LED 102.The figure is not drawn to scale in order to show better the structureof the device. The horizontal dimension of the QWIP-LED 102 isapproximately 1 cm and the dimension of the Sapphire 104 is slightlylarger. In the vertical direction the thickness of the QWIP-LED 102 isapproximately 3 μm and the thickness of the Sapphire 104 isapproximately 1 mm. The Sapphire 104 is connected to a cold finger 106,which is dimensioned such that efficient cooling is provided to keep theimaging device at a predetermined cryogenic operating temperature ofapproximately 65 K. In FIG. 1 a lens 108 interposed between the Sapphire104 and a CCD 110 is shown. Instead of the lens it is possible todirectly couple the light emitted from the LED to the CCD 110, forexample, via a fiber optical faceplate.

The detected infrared spectrum covers the middle and far infrared M/FIRwavelengths. The spectrum emitted by the LED is in the near infrared NIRor visible spectrum, which is possible to detect using a CCD.

Referring to FIG. 2 a simplified flow diagram of the principal steps ofa micro-fabrication method of pixelless infrared thermal imaging devicesaccording to the invention is shown. FIGS. 3a-3 n illustrateschematically resulting device structures corresponding to themanufacturing steps illustrated in FIG. 2. The micro-fabrication methodaccording to the invention allows the manufacture of numerous differentvariations of pixelless infrared thermal imaging devices based on theprincipal steps shown in FIGS. 2 and 3a-3 n. The fabrication of thevarious embodiments of imaging devices differs in the processesperformed within each of these principal steps. Moreover, it is possibleto change the order of some of the steps or to omit some steps as willbe described in the following.

In order to start crystal growth a crystal surface in the form of afirst substrate 1 is provided in a first step a). In the following stepsb) to e) a QWIP-LED wafer is crystallographically grown on the firstsubstrate 1. The growth of the QWIP-LED wafer starts with the depositionof material forming an etch stop layer 2, step b). The etch stop layer 2is followed by a bottom contact layer 3 formed in step c). In step d) aplurality of layers 4 forming the QWIP-LED are crystallographicallygrown on the bottom contact layer 3. In a final step e) the QWIP-LED 4is covered by a top contact layer 5.

Once the QWIP-LED wafer is grown a grating or V-grooves 6 are etchedinto the top layers of the wafer covering approximately the entireactive surface area of the device, as shown in step f). A device mesa 7comprising the active surface area is then etched—step g)—into the waferby removing the material outside the active area down to the bottomcontact layer 3. To facilitate electrical contacts an appropriate metal8, 9 is deposited in step h) on the top contact layer 5 outside theactive area near the edge of the device mesa 7 and on the bottom contactlayer 3. Following this, a thin coating 10 is deposited on the gratingsurface or the V-groove facets, shown in step i). Material defects 11included within the active area cause local shunts giving rise to acurrent and create a LED emission “hot spot”. These hot spots areremoved or isolated by short pulse laser ablation, shown in step j). Thedevice is then, step k), coupled to an optically transmissive materialsuch as a fiber optic faceplate 12 using, for example, an opticaladhesive 18. In step 1) the original substrate 1 is removed by acombination of polishing and etching. The etch is precisely stopped atthe etch stop layer 2. The etch stop layer 2 is then also removed—stepm). The device micro-fabrication is finished after etching of via holes13, 14 to the top 8 and bottom 9 contacts in step n). The device is thenready for mounting on a chip carrier and for wire bonding for electricalconnection.

FIG. 4 illustrates an example of grown layers forming a QWIP-LED wafer.The layers are, for example grown on a semi-insulating GaAs substrate 1using molecular beam epitaxy. As is evident, the fabrication methodaccording to the invention is not limited thereto and a person of skillin the art will find numerous methods applicable for growing the layers,for example, Metal Organic Chemical Vapor Deposition (MOCVD), as well asnumerous materials suitable as a first substrate. The first layer grownon the first substrate 1 is the etch stop layer 2. The etch stop layer 2shown in FIG. 4 comprises a 2500 Å thick layer of AlGaAs with an alloyfraction [Al]=45%. The etch stop layer 2 protects the bottom contactlayer 3 during the substrate removal process, step 1), which will bedescribed later. Thickness and material composition of the etch stoplayer 2 are chosen depending on the substrate removal process used. Forsome embodiments of imaging devices it is possible to omit the etch stoplayer 2. For example, imaging devices operating in a reflective mode asshown in FIG. 5a do not need removal of the first substrate 1.Furthermore, it is also possible to omit removal of the first substrate1 in imaging devices operating in a transmissive mode, shown in FIG. 5b,if an undoped substrate is used. In order to facilitate electricalconnection to the micro-fabricated device a n⁺ bottom contact layer 3 isgrown, step c), on the etch stop layer 2. The n⁺ bottom contact layer 3shown in FIG. 4 comprises a 7000 Å thick layer of GaAs. A stack oflayers forming multiple quantum wells of the QWIP follows the contactlayer. As shown in FIG. 4a the quantum wells comprise a 40 times repeatof a 350 Å thick i-AlGaAs barrier 4 a followed by a 49 Å thick Si centerdoped GaAs quantum layer 4 b giving rise to a two-dimensional carrierdensity of 5×10¹¹ cm⁻². Here, the GaAs layer is doped using Si in orderto form a n-QWIP. Alternatively, doping of the GaAs layer with Beprovides a p-QWIP as will described below. Growth is continued with theLED constituents: a 400 Å thick Al_(x)Ga_(1-x)As graded layer 4 c withx=0.24 at the beginning and decreasing to x=0.1 at the end followed by a300 Å thick GaAs well 4 d, a 400 Å thick Al_(x)Ga_(1-x)As graded layer 4e with x=0.1 at the beginning and increasing to x=0.24 at the end, a1000 Å thick p⁺—Al_(0.24)Ga_(0.76) layer 4 f doped to a Be gradedconcentration varying from 3×10¹⁸ cm⁻³ at the beginning to 10¹⁹ cm⁻³ atthe end, and a 500 Å thick p⁺—Al_(x)Ga_(1-x)As graded layer 4 g withx=0.24 at the beginning and x=0.14 at the end and doped to a Beconcentration of 10¹⁹ cm⁻³. The growth process is concluded by a 1000 Åthick p⁺—Al_(0.14)Ga_(0.86)As top contact layer 5 doped to a Beconcentration of 10¹⁹ cm⁻³ and a 150 Å thick p⁺—GaAs cap layer doped tothe same level.

The fabrication method according to the invention allows the manufactureof many variations, for example, by changing the QWIP quantum wellparameters such as materials used for growing the layers forming theQWIP, the thickness of each of the layers as well as the number ofrepeated layers. Furthermore, change of the LED layers, for example, useof InGaAs instead of GaAs, and change of the thickness of each of thelayers allows variation of the LED to have a different emissionwavelength in the NIR or visible spectrum and to fine tune transport andrecombination processes within the LED affecting overall performance ofthe device. For example, the exemplary thermal imaging device based onthe structure shown in FIG. 4 has a QWIP detection peak wavelength ofabout 9 μm and a LED emission wavelength of about 800 nm at a operatingtemperature of about 65 K.

Various embodiments of the growth process—steps a) to e)—of themicro-fabrication method according to the invention will be disclosed inthe following. Provision of a n⁺ substrate as the first substrate allowsomission of the etch stop layer 2 as well as the bottom contact layer 3for some thermal imaging devices such as, for example, imaging devicesbased on a reflective QWIP-LED geometry as shown in FIG. 5a. In anotherembodiment an additional layer, for example a 21000 Å thick AlGaAslayer, is grown on top of the top contact layer 5. This layer isdesigned to facilitate the fabrication of transmission grating couplers,which will be explained below. In another embodiment the thickness ofthe LED active region—layer 4 d—is increased in order to use photonrecycling effects for improving the external efficiency of the imagingdevice. However, carrier diffusion resulting in a spatial smearing of anincoming M/FIR image during its transformation into the NIR emissionrange limits the maximum thickness of the LED active region to about 1μm in order to provide an imaging device having a sufficient spatialresolution.

Referring to FIG. 6 another embodiment of a QWIP-LED wafer is shown. Thewafer comprises a p-type GaAs/AIGaAs QWIP combined with an InGaAs/GaAsLED. Use of a p-QWIP permits normal incidence excitation thus avoidingthe need for a grating coupler as required for n-type devices. Here, thebottom contact layer 3 comprises a p⁺—GaAs contact layer doped with Be.It is followed by the multiple quantum well growth comprising a repeatof a AlGaAs barrier 4 a followed by a Be center doped GaAs well 4 b.Growth is then continued with the LED constituents: a gradedAl_(x)Ga_(1-x)As layer 4 c with x=0.27 at the beginning and decreasingto x=0.1 at the end followed by a InGaAs well 4 d, a gradedAl_(x)Ga_(1-x)As layer 4 e with x=0.1 at the beginning and increasing tox=0.27 at the end, and a n⁺—Al_(0.27)Ga_(0.73) layer 4 f doped with Sito a concentration of 1.5×10¹⁸ cm⁻³. The growth process is concluded bya n⁺—GaAs top contact layer 5 again doped with Si to a concentration of1.5×10¹⁸ cm⁻³.

Referring to FIG. 7 yet another embodiment of a QWIP-LED wafer is shown.Here, the layers 3 to 5 shown in FIG. 4 are reversed, i.e. the bottomcontact layer comprises a p⁺ contact layer followed by the layersforming the LED. The growth process is then continued forming the layersof the QWIP and concluded by forming a n⁺ top contact layer.

FIG. 8 shows a QWIP-LED wafer comprising grown layers forming acombination of a n-QWIP and a p-QWIP with layers forming a LEDinterposed in between. Such a device is manufactured based on the samemicro-fabrication method as the embodiments disclosed above. As shownabove all layers are successively grown on a provided first substrateforming a QWIP-LED wafer. The combination of a n-QWIP and a p-QWIPallows sequential detection of M/FIR images at two different wavelengthsby switching the bias voltage between a high and a low value. Forexample, the imaging device based on the layers shown in FIG. 8 allowssequential detection of images at wavelengths of approximately 9 μm and5 μm.

N-type QWIPs respond mainly to the longitudinal component of the opticalelectric field, i.e. the field along the growth direction. Therefore, anoptical coupling structure, such as a diffraction grating or lamellarV-grooves, is required to scatter or diffract normally incident lightinto modes with an electric field component perpendicular to the quantumwells. FIGS. 9a and 9 b show schematically a grating for the QWIP-LEDwafer illustrated in FIG. 4. The left picture is an aerial view and theright picture illustrates a cross section. The dimensions shown in FIG.9a are suited for this particular wafer having a 9 μm wavelengthresponse. Using photo-lithography and etching, for example, chemicallyassisted ion beam etching, the grating is patterned into the top layersof the QWIP-LED wafer. As is obvious, many shapes and sizes are possiblesuch as for example, etching of a grid leaving elevated islands fordiffracting normally incident light. Furthermore, instead of an etchedgrating metal grids or metal dots are deposited on the top layer.Alternatively, a V-groove structure as shown in FIGS. 10a and 10 b ispatterned into the QWIP-layers of the QWIP-LED wafer. Experimentalresults showed that a V-groove structure etched through the LED activeregion leads to considerably lower Electro-Luminescent (EL) emissions ofthe LED. Therefore, it is preferred not to etch a grating or V-groovesinto the LED active region. In order to avoid surface contaminationprior etching of the fine structures it is preferred to perform thisstep immediately after the growth of the QWIP-LED wafer as shown FIG. 2.

In step g) a device mesa 7 comprising the active surface area is etchedinto the wafer by removing the material outside the active area down tothe bottom contact layer 3. The mesa area 7 is approximately the size ofa sensed 2-dimensional image. The mesa area 7 for imaging devicesproduced using the micro-fabrication method according to the inventionwas approximately 10.2 mm×10.2 mm and was etched using standard GaAslithography techniques. Of course, various sizes of the mesa area 7 arepossible to produce using the micro-fabrication method according to theinvention in order to meet application requirements.

To facilitate electrical connection to the top contact layer 5 anappropriate metal 8 is deposited in a narrow ring shape with a pad forwire bonding on the top part of the mesa 7 near the edge. Analogous, anappropriate metal 9 is deposited onto the bottom contact layer 3 in alarge area around the mesa 7. Appropriate metals are for example, TiPtAufor a p-type connection and sintered NiGeAu for n-type contacts.

In applications where the QWIP-LED wafer is used in an imaging deviceoperating in a transmissive mode M/FIR light is received at the bottomof the wafer and NIR light is provided through the top surface of thewafer, as shown in FIG. 11. A thin coating 10 is deposited on thegrating surface 6 to provide reflection in the M/FIR and at the sametime sufficient transmission in the NIR. Suitable coatings are, forexample, a thin gold film or a multi-layer dielectric Indium Tin Oxide(ITO) film. Thin layers of noble metals are good reflectors in the M/FIRand are partially transparent in the NIR. In the case of V-groovespatterned into the top layers of the wafer for bending the M/FIR lightthe V-groove facets are coated with a low index material having a smallabsorption coefficient in the M/FIR, for example CaF₂ or MgF₂. Thecoating minimizes M/FIR radiation absorption in the optical adhesivecaused by the fringing optical electrical field by optically decouplingthe GaAs from the adhesive. Even if the M/FIR light undergoes totalinternal reflection at a facet of the V-grooves, the optical electricalfield in fact extends beyond the GaAs/adhesive interface and is thenabsorbed in the adhesive.

Alternatively, when the QWIP-LED wafer is used in an imaging deviceoperating in a reflective mode—FIG. 5a—a coating being reflective forM/FIR as well as for NIR is preferred.

FIG. 12a shows a QWIP-LED wafer having a material defect 11 includedwithin the active area. In the production of large area devices it isnext to impossible to avoid inclusion of one or more material defectswithin the active area without substantially increasing manufacturingcosts by either using more expensive and/or time consuming manufacturingprocesses or by discarding a majority of the production due to materialdefects. The material or crystallographic defects locally short circuitthe large area device at low temperature causing a local shunt. Thelocal shunt is giving rise to a current creating a LED emission “hotspot”, which is considerably disturbing the NIR image provided by theQWIP-LED wafer. Typically, devices produced using currently availableproduction techniques have approximately 30 hot spots/cm². The hot spotsare removed by isolating the material defects using short pulse laserablation, for example, femtosecond laser ablation at a temperature of 63K. FIGS. 12b and 12 c show the isolation of material defects bypatterning a trench surrounding the top portion of the defect or byremoving the top portion of the defect by patterning a crater usingshort pulse laser ablation. Details concerning the short pulse laserablation technique are disclosed by the inventor in Provisional U.S.Patent Application No. 60/177,674, and in E. Dupont, X. Zhu, S. Chiu, S.Moisa, M. Buchanan, M. Gao, H. C. Liu, P. B. Corkum, Semiond. Sci.Technol. 15, L15 (2000).

The QWIP-LED wafer is then bonded to an optical faceplate such as aSapphire plate or a fiber optical face plate using an optical adhesive,for example, UV and/or heat cured epoxy. Optionally, the QWIP-LED waferis bonded direct to a NIR detector such as a CCD using a heat curedadhesive because both GaAs and Si forming the CCD are opaque to UVlight. Requirements for the adhesive include: optical transparency atthe LED emission wavelength, good long-term performance at cryogenictemperatures, tolerance to thermal cycling, resistance to etchants andsolvents, strong bond formation between the faceplate and GaAs, and goodcuring behaviors such as low shrinkage. Since the QWIP-LED waferoperates at temperatures <80 K to reduce dark current, the opticaladhesive is required to retain its optical and structural integrity atcryogenic temperatures. Also, since device fabrication steps includeprocessing on the opposite side of the wafer the adhesive is exposed toa photoresist bake temperature of ˜120° C. Furthermore, low shrinkageduring the curing process minimizes the strain acting on the device.

After bonding of the wafer to the optical faceplate but before curing ofthe adhesive the unbonded side of the wafer is moved into anapproximately parallel orientation to the unbonded side of thefaceplate. Preferably, in order to achieve a higher order of parallelismand to control the thickness of the adhesive a contact mask aligner isused to press the wafer to the faceplate.

Optionally, the bonded side of the faceplate is coated with a dielectriccoating having a refractive index being between the refractive index ofthe faceplate and the refractive index of the adhesive for thewavelength of the LED emission in order to reduce Fabry Peyrotreflection due to unparallelism between the bonded side of the faceplateand the bonded side of the wafer.

When the wafer is bonded directly to a CCD the effect of the adhesive onthe escape probability of the LED emission as a result of the differencein indexes of refraction between the adhesive and the GaAs is minimizedif the thickness of the optical adhesive layer is less than thewavelength of the LED emission. The bonding strength of such a thinadhesive layer is still sufficient.

During the bonding process care is taken in order to avoid incorporationof bubbles in the adhesive to prevent complications associated withstress induced by air bubbles shrinking and expanding during thermalcycling. Often visible air bubbles are observed in the adhesive aftermixing. It is, therefore, preferred to use an adhesive that does notrequire a mixing step. It has been observed that bubbles are alsoincorporated into the adhesive at the moment of bonding. Using a specialdeveloped “leveler” to slowly and reproducibly bring the surfaces, whichare to be bonded, in contact with the adhesive, eliminated the bubbleinclusion.

After the full cure, the first substrate 1 of the QWIP-LED is polishedto a thickness of ˜80-50 μm using precision lapping and polishingmachines. For example, a 3 μm slurry is first used to grind thesubstrate down to ˜60 μm and then a 0.3 μm slurry is used to polish offan additional 5-10 μm and to provide a mirror like surface. Optionally,if the 3 μm slurry is used it is possible to skip the 0.3 μm step, sincethe etch stop layer 2 will tolerate a 3 μm surface roughness. Theremaining ˜50 μm substrate 1 is then removed using wet etch techniques.The substrate surface is first etched by a 45 s dip in a 1:10 NH₄OH:H₂Osolution to remove surface oxides followed by a 4:1 citric acid solution(1:1 citric acid: H₂O): hydrogen peroxide solution (30% H₂O₂) of wetetch to completely remove the substrate. The etch is precisely stoppedat the etch stop layer 2. The etch stop layer 2 first grown on the firstsubstrate 1 is determined to tolerate small thickness variations—up to10 μm—in the remaining ˜80—50 μm substrate after polishing. The etchstop layer 2 is then also removed using concentrated hydrofluoric acid(49%). Of course, numerous other methods to remove the substrate areapplicable as is evident to the person of skill in the art. For example,it is possible to skip the polishing step and to remove the wholesubstrate using an etching technique. However, this process has thedisadvantage that it requires long processing times for completelyremoving the substrate. More details concerning the substrate removalare disclosed by the inventor in S. Chiu, M. Buchanan, E. Dupont, C. Py,H. C. Liu, Infrared Physics & Technology 41 (2000) 51-60.

Referring to FIG. 13, an alternative embodiment of the micro-fabricationmethod according to the invention is shown. Here, the step f) of etchingof a grating into the top layers is replaced by etching the grating intothe bottom layers—the entrance side of the M/FIR radiation—of theQWIP-LED wafer after substrate removal. The M/FIR entrance side is thenbonded using a M/FIR non-absorbing adhesive to a M/FIR transparentsubstrate such as GaAs or ZnSe. In these applications the grating worksin a transmissive mode.

To improve the LED efficiency, a coating on the entrance side of theM/FIR bottom side of the wafer—is deposited. The coating is transmissivein the M/FIR and reflective in the NIR to visible spectrum. For example,a stack of 8 layers of ThF₄/ZnSe with total thickness of ˜1.1 μm has therequired characteristics.

It is also possible to incorporate the reflector during epitaxialgrowth, for example, by growing a distributed Bragg reflector (DBR)after growth of the etch stop layer 2 and before growing the bottomcontact layer 3. Preferably, the distance between the LED active regionand the first layer of the DBR is chosen to be half the LED emissionwavelength in order to fully use interference effects.

Optionally, the substrate removal process is obviated if V-grooves andan absorbing semiconductor substrate at the LED wavelength—for example,GaAs LED and a GaAs substrate—are combined. Furthermore, using V-groovesabove the active layer of the LED act as a microlens and, therefore,enhance the LED emission.

As is evident, the micro-fabrication method according to the inventionallows manufacture of various different thermal imaging devices usingnumerous different material systems such as an InGaAs well combined withan AlGaAs barrier or an InGaAs well combined with an InP barrier, bothgrown on InP substrates. This allows manufacture of QWIP-LEDs operatingat different detection wavelengths as well as different emissionwavelengths.

The micro-fabrication method according to the invention is highlyadvantageous by allowing manufacture of numerous different infraredthermal imaging devices based on epitaxial integration of a QWIP with aLED. The various steps of the micro-fabrication method is based onstandard manufacturing techniques, for example, epitaxial growth andetching. Furthermore, various different devices are manufactured bychanging the order of manufacturing steps, omitting some steps or usingdifferent materials. Therefore, it is possible using a samemanufacturing equipment for producing a large variety of differentdevices considerably reducing manufacturing costs.

FIGS. 14a to 14 m illustrate schematically the principal structure ofvarious examples of imaging devices manufactured using themicro-fabrication method according to the invention. The examplesillustrated in FIGS. 14a to 14 d are produced based on themicro-fabrication method according to the invention shown in FIG. 2 andsome modifications of this method as described above. The examplesillustrated in FIGS. 14e to 14 f are based on the micro-fabricationmethod shown in FIG. 13. The examples illustrated in FIGS. 14g to 14 kare again based on the method shown in FIG. 2 replacing the gratingswith V-grooves, wherein examples illustrated in FIGS. 14h and 14 j thestep of substrate removal has been omitted. Examples illustrated inFIGS. 14l and 14 m are produced based on a variation of the method shownin FIG. 13. Here the operation of the device is reversed, i.e. the M/FIRradiation is received at the top and the NIR emission is transmittedthrough the bottom of the device. This necessitates bonding of theQWIP-LED wafer to a transparent substrate at the top in order to enableremoval of the first substrate at the bottom, followed by etchingV-grooves into the bottom layers and then bonding the bottom side of thewafer to an optical faceplate.

Numerous other embodiments of the invention will be apparent to personsskilled in the art without departing from the spirit and scope of theinvention as defined in the appended claims.

What is claimed is:
 1. A method for micro-fabricating a pixellessthermal imaging device, the imaging device for up-converting a sensed2-dimensional M/FIR image into a 2-dimensional image in the NIR tovisible spectrum in dependence thereupon, the method comprising thesteps of: providing a first substrate, the first substrate having asurface suitable for subsequent crystal growth; crystallographicallygrowing an integrated QWIP-LED wafer on the surface of the firstsubstrate comprising the steps of: growing an etch stop layer; growing abottom contact layer; growing a plurality of layers forming a n-typeQWIP and a LED; and, growing a top contact layer; providing to theQWIP-LED wafer an optical coupler for coupling at least a portion ofincident M/FIR light into modes having an electric field componentperpendicular to quantum wells of the QWIP; removing the firstsubstrate; and, removing the etch stop layer.
 2. A method formicro-fabricating a pixelless thermal imaging device as defined in claim1, wherein the bottom contact layer comprises a n⁺ contact layer andwherein the top contact layer comprises a p⁺ contact layer.
 3. A methodfor micro-fabricating a pixelless thermal imaging device as defined inclaim 2, wherein at least a layer forming the LED is grown on top of thelayers forming the QWIP.
 4. A method for micro-fabricating a pixellessthermal imaging device as defined in claim 1, wherein the bottom contactlayer comprises a p⁺ contact layer and the top contact layer comprises an⁺ contact layer.
 5. A method for micro-fabricating a pixelless thermalimaging device as defined in claim 4, wherein the layers forming theQWIP are grown on top of the at least a layer forming the LED.
 6. Amethod for micro-fabricating a pixelless thermal imaging device asdefined in claim 1, comprising the step of growing a distributed Braggreflector.
 7. A method for micro-fabricating a pixelless thermal imagingdevice as defined in claim 6, wherein a distance between an activeregion of the LED and a first layer of the Bragg reflector isapproximately half the wavelength of the LED emission.
 8. A method formicro-fabricating a pixelless thermal imaging device as defined in claim1, wherein the QWIP-LED wafer is grown using molecular beam epitaxy. 9.A method for micro-fabricating a pixelless thermal imaging device asdefined in claim 1, wherein the QWIP-LED wafer is grown using metalorganic chemical vapor deposition.
 10. A method for micro-fabricating apixelless thermal imaging device as defined in claim 1, comprising thestep of isolating material defects in an active area of the QWIP-LED,the active area being approximately the size of the 2-dimensional image.11. A method for micro-fabricating a pixelless thermal imaging device asdefined in claim 10, comprising the step of patterning a device mesa byremoving the layers outside the device mesa down to the bottom contactlayer, the device mesa approximately comprising the active area of thethermal imaging device.
 12. A method for micro-fabricating a pixellessthermal imaging device as defined in claim 10, wherein the device mesais patterned using etching a lithography techniques.
 13. A method formicro-fabricating a pixelless thermal imaging device as defined in claim10, comprising the steps of: depositing a top metal contact onto the topcontact layer; and, depositing a bottom metal contact onto the bottomcontact layer outside the device mesa.
 14. A method formicro-fabricating a pixelless thermal imaging device as defined in claim13, comprising the step of providing vias to the top and bottom metalcontacts.
 15. A method for micro-fabricating a pixelless thermal imagingdevice as defined in claim 14, comprising the step of bonding the top ofthe QWIP-LED wafer to an optical faceplate such that the QWIP-LED is inoptical communication with the optical faceplate for light emitted fromthe LED.
 16. A method for micro-fabricating a pixelless thermal imagingdevice as defined in claim 15, wherein the bonding is provided using anoptical adhesive.
 17. A method for micro-fabricating a pixelless thermalimaging device as defined in claim 16, wherein the surface of theoptical faceplate being in contact with the optical adhesive is coatedwith a dielectric coating prior to the bonding, the dielectric coatinghaving a refractive index between the refractive index of the opticalfaceplate and the refractive index of the optical adhesive.
 18. A methodfor micro-fabricating a pixelless thermal imaging device as defined inclaim 1 comprising the steps of: patterning a device mesa by removingthe layers outside the device mesa down to the bottom contact layer, thedevice mesa approximately comprising an active area of the thermalimaging device, the active area being approximately the size of the2-dimensional image; depositing a top metal contact onto the top contactlayer such that the top metal contact forms a ring surrounding theactive area; depositing a bottom metal contact onto the bottom contactlayer outside the device mesa; depositing a coating onto the top surfaceof the active area; isolating material defects in the active area of theQWIP-LED; and, bonding the QWIP-LED wafer to an optical faceplate suchthat the QWIP-LED is in optical communication with the optical faceplatefor light emitted from the LED.
 19. A method for micro-fabricating apixelless thermal imaging device as defined in claim 18, wherein theQWIP comprises a stack of layers forming multiple quantum wells.
 20. Amethod for micro-fabricating a pixelless thermal imaging device asdefined in claim 19, comprising the step of growing a coupler layer ontop of the top contact layer.
 21. A method for micro-fabricating apixelless thermal imaging device as defined in claim 19, wherein theoptical coupler comprises a diffraction grating.
 22. A method formicro-fabricating a pixelless thermal imaging device as defined in claim21, comprising the step of depositing a reflective coating onto thegrating surface, the coating being reflective in the M/FIR andsufficiently transmissive in the NIR.
 23. A method for micro-fabricatinga pixelless thermal imaging device as defined in claim 21, comprisingthe step of depositing a reflective coating onto the grating surface,the coating being reflective in the M/FIR and being reflective in theNIR.
 24. A method for micro-fabricating a pixelless thermal imagingdevice as defined in claim 19, wherein the optical coupler compriseslamellar V-grooves.
 25. A method for micro-fabricating a pixel lessthermal imaging device as defined in claim 24, comprising the step ofdepositing a coating onto the V-groove facets, the coating comprising alow index material having a small absorption coefficient in the M/FIR.26. A method for micro-fabricating a pixelless thermal imaging device asdefined in claim 21, wherein the grating is patterned into the top ofthe QWIP-LED wafer using photo-lithography and etching.
 27. A method formicro-fabricating a pixelless thermal imaging device as defined in claim26, wherein the grating is patterned by patterning a grid into the topleaving elevated islands.
 28. A method for micro-fabricating a pixellessthermal imaging device as defined in claim 19, wherein the opticalcoupler comprises a metal structure deposited onto the top surface. 29.A method for micro-fabricating a pixelless thermal imaging device asdefined in claim 19, comprising the step of depositing a coating ontothe bottom side of the QWIP-LED wafer, the coating being transmissive inthe M/FIR and reflective in the NIR.
 30. A method for micro-fabricatinga pixelless thermal imaging device as defined in claim 1 comprising thesteps of: patterning a device mesa by removing the layers outside thedevice mesa down to the bottom contact layer, the device mesaapproximately comprising an active area of the thermal imaging device,the active area being approximately the size of the 2-dimensional image;depositing a top metal contact onto the top contact layer such that thetop metal contact forms a ring surrounding the active area; depositing abottom metal contact onto the bottom contact layer outside the devicemesa: isolating material defects in the active area of the QWIP-LED;bonding the top surface of the QWIP-LED wafer to an optical faceplatesuch that the QWIP-LED is in optical communication with the opticalfaceplate for light emitted from the LED; and, bonding the QWIP-LEDwafer to a plate such that the QWIP-LED is in optical communication withthe plate for M/FIR light.
 31. A method for micro-fabricating apixelless thermal imaging device as defined in claim 30, wherein the LEDcomprises an active layer having sufficient thickness for photonrecycling.
 32. A method for micro-fabricating a pixelless thermalimaging device as defined in claim 31, wherein the thickness of theactive layer does not exceed a predetermined limit, the limit beingdetermined based on a required spatial resolution of the imaging device.33. A method for micro-fabricating a pixelless thermal imaging device asdefined in claim 30, wherein the material defects are isolated usingshort pulse laser ablation.
 34. A method for micro-fabricating apixelless thermal imaging device as defined in claim 33, wherein thefirst substrate is removed using a wet etching technique.
 35. A methodfor micro-fabricating a pixelless thermal imaging device as defined inclaim 34, comprising the step of polishing the first substrate prior tothe wet etching.
 36. A method for micro-fabricating a pixelless thermalimaging device as defined in claim 1, wherein the optical coupler isprovided at the top of the QWIP-LED wafer.
 37. A method formicro-fabricating a pixelless thermal imaging device as defined in claim30, wherein the optical coupler is provided at the bottom of theQWIP-LED wafer.
 38. A method for micro-fabricating a pixelless thermalimaging device as defined in claim 37, wherein the bottom of theQWIP-LED wafer is bonded to the plate.